A composite is a material produced by mixing or combining two or more independent components or phases in various ways. Each of the components has a content at a significant level, i.e. 5% or more. It this case, the components have different properties. Thus, the composite as a mixture of the components has properties different from those of each component. That is, the composite is a combination of chemically independent a plurality of phases at a microscopic level, and it is very important to identify properties of each phase.
The components of the composite include a matrix and a stiffener. The matrix is a component in the form of a continuum and takes up a considerable part of the total volume of the composite and determines a macroscopic shape of the composite. Further, the stiffener dominates strength and stiffness of the composite and the stiffener's form is maintained by the matrix. A representative stiffener is fiber.
In most cases, a composite comprised of such components is lighter, stronger and less easily transformed than a matrix due to an effect of a stiffener. It can be seen that a mechanical property of the composite is determined depending on a function of shapes and contents of the matrix and the stiffener.
Methods to reinforce a material in the form of a composite are roughly two methods: a fibrous method and a particulate method. The fibrous method uses continuous fiber or discontinuous fiber having a very long length as compared with a cross-sectional dimension. The particulate method uses particles with similar dimensions in all directions. By way of example, particles having a regular shape, such as a circle, a hexahedron, and the like, or an irregular shape may be used in the particulate method.
A composite may be formed in a single layer or a plurality of layers. Multi-layer composites include laminates, hybrid composites, and the like. The laminates have a structure of the plurality of layers (plies or laminas) stacked in a specific sequence. Further, the hybrid composites are composites produced from many kinds of fibers and can be produced by layering, or mixing or combining, for example, glass fibers and carbon fibers.
However, such a composite has complicated damage mechanisms depending on repeated loadings and the number of repeated loadings as compared with a metal material, and it is difficult to observe such complicated damage from the outside. By way of example, in a composite, complicated damage such as fiber breakage, matrix cracking, ply delamination, fiber/metal delamination, plastic deformation, and the like may occur. Further, a composite is very vulnerable to an impulsive load, and, thus, such damage frequently occurs. Furthermore, if damage occurs from the inside, its load supporting capacity is sharply decreased.
Such a composite is contained in a structure, bodywork, a load supporting body, e.g. cars, railroad cars, airplanes, ships, buildings, wind turbine blades, and the like, it is not easy to recognize whether or not internal damage occurs.